1. Field of the Invention
The present invention relates to an electric power steering apparatus for an automobile.
2. Related Background Art
Steering systems of automobiles widely use a so-called power steering apparatus for assisting a steering operation by use of an external power source. the power source for the power steering apparatus has hitherto involved the use of a vane hydraulic pump, and this hydraulic pump is driven by the engine in many cases. This type of power steering apparatus has, however, a large drive loss (on the order of several through ten horsepower (HP) at a maximum load) of the engine because of driving the hydraulic pump at all times and is therefore hard to apply to mini-sized motor vehicle having small displacements. Even the automobiles having comparatively large displacements, when installed, come to have such an inevitable demerit that their traveling fuel efficiencies become low enough not to be ignorable.
Such being the case, an electric power steering (which will hereinafter be abbreviated to EPS) involving the use of an electric motor as a power source has attracted the attention over the recent years for obviating those problems. The EPS apparatus exhibits characteristics, wherein there is no direct drive loss of the engine because of using a battery loaded into the car as a power source for the electric motor, a decrease in the traveling fuel efficiency is restrained as the electric motor is started only when assisting the steering operation, and in addition the electronic control can be extremely easily performed.
On the other hand, a steering gear for the automobile is high in rigidity and low in weight, and hence a rack-and-pinion steering gear is mainly employed at the present. Then, the EPS apparatuses for the rack-and-pinion steering gear include a column assist type in which the electric motor is disposed sideways of the column in order to drive a steering shaft and a pinion itself, and in addition a ball screw rack assist type in which a rack shaft is driven by an electrically-driven ball screw mechanism. In the ball screw rack assist type EPS apparatus (which will hereinafter be simply called the rack assist type EPS apparatus), the assist force does not act on a meshing surface between the pinion and the rack, and therefore a contact surface pressure between these two members, which is to be a factor for abrasion and deformation as well, is comparatively small.
According to the rack assist type EPS apparatus, a ball screw shaft male thread groove formed in a rack shaft engages with a female thread groove formed in a ball nut through a multiplicity of circulation balls (steel balls), and the ball nut is rotated by the electric motor, thereby moving the rack shaft in the axial directions. A power transmission method for transmitting the power from the electric motor to the ball nut may be a timing belt system disclosed in Japanese Utility Model Publication No.5-14939 (a first prior art) etc, however, a general method is a gear system disclosed in Japanese Utility Model Publication No.2-46455 (a second prior art) etc.
(1) In a rack assist type EPS apparatus, an electric motor is installed by using bolts to a steering gear case, and the installing mode thereof is classified into a one-point support type and a two-point support type. For example, according to the second prior art, the electric motor is fastened at its one end by bolts to a ball screw mechanism housing protruding from the steering gear case. Further, according to the first prior art, the electric motor is fastened at its one end by bolts to the ball screw mechanism housing and further fastened at the other end likewise by bolts to the side surface of the steering gear case.
By the way, the rack assist type EPS apparatus adopts in many cases the electric motor that is small in its major diameter and comparatively large in its total length in terms of a space for installation for the convenience's sake of being attached to the steering gear disposed at a lower portion of the car body. Therefore, if the electric motor is attached in the one-point support mode to the steering gear case, an inertial moment is large due to vibrations caused when traveling on a rough road etc, and an excessive bending stress etc acts on the steering gear case, with the result that the steering gear case might be deformed and damaged. Then, when scheming to increase a rigidity and a strength of the electric motor fitting portion (the ball screw mechanism housing etc) of the steering gear case in order to prevent the deformation, damage and so on, increases in frame size and in weight are inevitable from a necessity of obtaining an increased wall thickness of this fitting portion.
While on the other hand, in the case of fitting the electric motor to the steering gear case in the two-point support mode, if the steering gear case gets flexural due to the vibrations caused when traveling and if the electric motor undergoes a thermal expansion due to frequent actuations thereof, a distance between the supporting or fitting portions to the steering gear case is not coincident with a total length of the electric motor. As a result, a compression stress or a tensile stress acts on the electric motor and on the steering gear case, with the result that the electric motor might fall into a failure and the steering gear case might be deformed and damaged. Further, in the case of taking a structure in which the steering gear case and the electric motor are disposed horizontally in a way that support the side surface of the electric motor with a stay in order to ensure a minimum road clearance in the electric power steering apparatus, dusts and mud accumulate in between the steering gear case and the electric motor and on an upper surface of the shelved stay, and then contain a water content of rain, resulting in rusting of each of the constructive members as the case may be.
(2) A rack assist type EPS apparatus, of which an electric motor is disposed on a axis different from the ball nut, uses the electric motor that is larger in output and in size than a coaxially-disposed electric motor, and hence it is difficult to ensure a heat-radiation air space in periphery of the electric motor. Consequently, a temperature of the electric motor receiving the heat from the engine, an exhaust pipe etc, rises depending on where the electric motor is installed, and a predetermined output might not be obtained.
It is desirable for preventing this rise in temperature that the electric motor be installed as far away from the heat source as possible. To be specific, since the engine and the exhaust pipe are disposed comparatively upward, a lower portion of the engine room is highly available for installing the electric motor. In the case of installing the electric motor in the vicinity of the steering gear case, however, it is required that the minimum ground clearance should not be reduced to the greatest possible degree in order to avoid interference with an obstacle when traveling on the rough road.
(3) In a rack assist type EPS apparatus with an electric motor disposed on an axis different from the ball nut, it follows that the electric motor is positioned in the lower portion of the car body, which is comparatively close to the road surface. It is therefore inevitable that the water is splashed over the electric motor and an electric harness thereof when traveling in the rain and passing through the puddles and that the pebbles rebounding from the tires when traveling on the rough road impinge thereon.
As known well, the output (torque generated) of the electric motor changes depending on its temperature, and hence, when the electric motor is abruptly cooled by the splashed water, it follows that the steering assist force of the EPS apparatus changes abruptly. This has been a factor for hindering smooth steering by the driver and making a steering feeling unagreeable. Further, the impingement of the pebbles upon the electric motor, the electric harness, a connector and so forth, has been a factor for rusting due to exfoliation of coating and plating on the motor body and for causing a disconnection and a short-circuit of the electric circuit. On the other hand, a housing generally composed of an aluminum alloy etc accommodates a gear train and a belt defined as a power transmission mechanism between the electric motor and the power assist mechanism, but might be cracked by the impingement of the rebounding pebbles, with the result that a lubricant might leak out and the muddy water might permeate, and so on.